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Feline acromegaly: The keys to diagnosis

Caused by excessive growth hormone secretion, this likely underdiagnosed endocrinopathy may be lurking in your feline patients—especially older, poorly controlled diabetic males. Here's a look at which diagnostic tests can help you detect it.

Feline acromegaly is a disease characterized by excessive growth hormone secretion, leading to a wide array of clinical signs caused by the hormone's effects on multiple organ systems. These effects can be divided into two major classes: catabolic and anabolic. The catabolic actions of growth hormone include insulin antagonism and lipolysis, with the net effect of promoting hyperglycemia. The slow anabolic (or hypertrophic) effects of growth hormone are mediated by insulin-like growth factors.

Growth hormone stimulates the production of insulin-like growth factors in several tissues throughout the body. Insulin-like growth factor-1 (IGF-1), which is produced in the liver, is thought to be the key factor that facilitates the anabolic effects of growth hormone that are responsible for the characteristic appearance of people, dogs, and cats with acromegaly.

Similar to its etiology in people, acromegaly in cats is the result of a functional adenoma of the pituitary gland that releases excessive growth hormone despite negative feedback.1

ANATOMY AND PHYSIOLOGY

Growth hormone is produced in an anterior lobe of the pituitary gland, specifically by cells called somatotrophs. The regulation of growth hormone is complex, and many factors—both environmental and endogenous—are responsible for its control. The two most important regulators of growth hormone production and release are growth hormone-releasing hormone (GHRH) and somatostatin, which are produced in the hypothalamus. While growth hormone release is stimulated by GHRH, it is inhibited by somatostatin as well as by negative feedback from itself and IGF-1.1

SIGNALMENT

Feline acromegaly is an uncommon disease, although it is thought to be underdiagnosed. It most commonly affects middle-aged and older, male castrated cats. In one study, 13 of 14 cats with acromegaly were males, with an average age of 10.2 years.2 This association may be biased, however, as most cats in which acromegaly is diagnosed are presented for complications associated with diabetes mellitus, which is also common in older, male castrated cats. Based on available data, no known breed association for feline acromegaly exists.

CLINICAL SIGNS

1. This domestic shorthaired cat with presumptive acromegaly is exhibiting a broadened face, a physical change commonly associated with feline acromegaly. The cat was presented for unregulated diabetes.

Cats with acromegaly are commonly presented for insulin-resistant diabetes mellitus (insulin doses dependent on insulin type) with concurrent weight gain rather than weight loss.2 Other clinical signs vary because of the wide range of effects the disease has on the body.

2. The same cat as in Figure 1 exhibiting another physical change associated with feline acromegaly—protrusion of the mandible.

Respiratory disease may result from excessive growth of the soft palate and laryngeal tissues, leading to stertorous breathing and even upper airway obstruction. Cardiovascular signs include the presence of a heart murmur, hypertension, arrhythmia, and congestive heart failure associated with hypertrophic cardiomyopathy.3

Neurologic disease associated with feline acromegaly is uncommon but can occur with large pituitary adenomas. Neurologic signs that have been observed with acromegaly include dullness, lethargy, abnormal behavior, circling, and blindness.

3. This close-up of the cat's teeth (the same cat as in Figures 1 & 2) highlights increased interdental spacing, another physical change associated with feline acromegaly.

Glomerulopathy and secondary renal failure have also been associated with feline acromegaly. Histologic evaluation of the kidneys of cats with acromegaly has revealed thickening of the glomerular basement membrane and Bowman's capsule, periglomerular fibrosis, and degeneration of the renal tubules.2

Because of an associated degenerative arthropathy and peripheral (diabetic) neuropathy, lameness has also been noted in cats with acromegaly.